1. Field of the Invention
The present invention relates to a power supply apparatus such as an AC adapter, and more particularly to a power supply apparatus which not only supplies a drive current to a battery operable electronic apparatus such as a personal computer but also operates as a charger for supplying a charging current to a driving battery.
2. Description of Related Art
FIG. 7 shows an example of conventional construction. In the figure, an AC adapter 1 converts an AC input to DC and outputs it. Voltage feedback control is performed in the inside of the AC adapter 1 and constant voltage output is produced from the AC adapter 1. Connected to a battery operable electronic instrument 2 such as a personal computer is a battery 3. The electronic instrument 2 includes a diode 4, a DC voltage converter 5, and a system load 6, and the constant voltage output of the AC adapter 1 is converted to DC voltage suitable for the operation of the system Load 6 by the DC voltage converter 5 and then is provided to the system load 6. If the electronic instrument 2 is operated by a battery, the output of the battery 3 is provided to the DC voltage converter 5 through an FET switch 7.
Further, the electronic instrument 2 includes an internal charger 8 to which the constant voltage output of the AC adapter 1 is also provided. The internal charger 8, which is a power supply apparatus for outputting a constant current, supplies a charging current to the battery 3. If the output of the AC adapter 1 is supplied direct to the battery 3 without using the internal charger 8, there is a risk that excessive current flows into the battery 3 since the output of the AC adapter 1 is not a constant current. Therefore, such construction cannot be adopted.
In the conventional construction of FIG. 7, the AC adapter 1 can supply a charging current to the battery 3 through the internal charger 8 while it supplies a drive current to the system Load 6. That is, the battery 3 can be charged while the electronic instrument 2 is working. However, when importance is attached to the portability of the battery operable electronic instrument 2, it is difficult to make the internal charger 8 of a large capacity output, that is, a large size in respect of mounting it. Therefore, the value of the outputted current (charging current) of the internal charger 8 cannot but become small, and it was impossible to charge in a short time. Further, regardless of the current value of the constant voltage output of the AC adapter 1, the current value of the output (charging current) of the internal charger 8 is fixed. Therefore, even if the power consumption of the system load 6 is small, the value of the charging current is invariable, and the charging time cannot be accordingly shortened simply because the power consumption of the system load 6 is small.
FIG. 8 shows another example of conventional construction. In the example, a current sensor 9 is provided for detecting the current value of the constant voltage output of the AC adapter 1. Further, the constant current output value of an internal charger 10 varies according to a detected value by the current sensor 9. For example, when an electronic instrument 11 normally operates and the current value of the constant voltage output of the AC adapter 1 is within the range from 0.15 to 1.7 amperes, the internal charger 10 charges the battery 3 by the constant current of 0.60 amperes, and when the current value of the constant voltage output of the AC adapter 1 is within the range from 0 to 0.15 amperes, the internal charger 10 charges the battery 3 by the constant current of 1.85 amperes.
Also in such conventional construction of FIG. 8, in a current value within each range described above, the current value of the output (charging current) of the internal charger 10 is fixed regardless of the current value of the constant voltage output of the AC adapter 1. Therefore, simply because the power consumption of the system load 6 is small while the electronic instrument 11 normally operates, the surplus power of the AC adapter 1 cannot be utilized as a charging current and the charging time cannot be accordingly shortened. Further, in the conventional construction of FIG. 7 and FIG. 8, since the AC adapter 1 itself outputs only a current of a constant voltage, the internal chargers 8 and 10 should be provided inside the electronic instrument 2 so as to charge the battery 3, but it is disadvantageous to provide the internal chargers 8 and 10 in enhancing the portability of the electronic instruments 2 and 11.
FIG. 9 shows still another example of conventional construction. In the figure, an AC adapter 14 has construction in which a constant voltage output apparatus and a constant current output apparatus are combined and has an output line for a constant voltage and an output line for a constant current separately. An internal charger is not provided to an electronic instrument 15, and the constant current output line of the AC adapter 14 is connected to the battery 3 through a switch 16. The constant voltage output line is connected to the DC voltage converter 5. In the conventional construction of FIG. 9, by providing a constant current output function having a sufficient capacity to the AC adapter 14, the battery 3 can be charged in a short time while the electronic instrument 15 operates. However, since power of two systems is provided to the AC adapter 14, construction becomes complicated and large-sized. Further, since three power connecting lines become necessary between the AC adapter 14 and the electronic instrument 15, also in this respect, construction becomes complicated and large-sized.